During mineral extraction from the earth, metals such as iron, aluminum, and manganese are exposed to the atmosphere and moisture. As these metals become insoluble, hydrogen ions and acidity are produced. When the pH is lowered it enhances the dissolution of surrounding strata whereby more metals are leached into the ground waters. Metal concentrations are common in acid mine water; with the greatest concentrations being iron, aluminum, and manganese. The metals in their reduced state act as oxygen sinks and deplete from the water the dissolved oxygen that is necessary for a healthy aquatic ecosystem. It is not uncommon for acid mine water to contain less than 1 mg/l of dissolved oxygen.
Passive treatment is one method of treating the acid mine water. This requires the use of lagoons or engineered wetlands that depend on oxygen diffusion from the atmosphere; thus engineering criteria for wetland design often include minimal depth and a large surface area. Since only the top few inches of water can absorb oxygen, it takes a long time for soluble iron to be removed from the water. Large land masses at many mine discharge sites are not available, thereby making the removal of soluble metals from the water by passive treatment impossible. Finally, the passive treatment system cannot remove the last 10 mg/l of iron in the water, nor will it remove manganese. Most existing active treatment processes require less land but are labor intensive and have higher costs associated with them. In active treatment systems the biggest obstacles to overcome are the ability to adjust the water pH efficiently and to add oxygen to the water.
To adjust the pH, slaked lime is injected into the acid mine water until the pH is about 7.0. The slaking process requires expensive equipment and the maintenance of the equipment and associated pipes is labor intensive. Although lime treatment adjusts the pH easily only about 10 percent of the lime is consumed during the adjustment process and the remaining 90 percent becomes a waste disposal problem as it cannot be reused.
After the pH has been adjusted the water is pumped to an aeration tank where air is forced into the bottom of the tank and passed through a porous stone. This breaks the air into small bubbles. As the bubbles rise to the surface of the water, oxygen is absorbed by the acid mine water and the insoluble iron is oxidized. The water cannot become saturated with dissolved oxygen by this method. This type of treatment takes a relatively long time for the metals to precipitate.
This means the settling ponds, aerators, and clarifiers could be very large. The acid mine water demineralizer apparatus is small in size, 16 inches in diameter and 36 inches long and is designed to handle flow rates ranging from 500 to 2,000 gallons per minute per apparatus. Multiple demineralizers can be connected in parallel so that the amount of water to be processed could easily exceed 10,000 gpm.
The demineralizer saturates the oxygen depleted water with dissolved oxygen in milliseconds. Sharp edged orifices in the apparatus give the demineralizer the ability to oxidize metals by 4 orders of magnitude (or greater than literature obtained oxidation rates). The use of ozone has the added advantage of removing aluminum and manganese from the water at the same time the iron is being oxidized. Approximately 50 percent of the insoluble metals are oxidized within one second.